Conventional lead-acid batteries contain a positive electrode (PbO.sub.2 plate) and a negative electrode (Pb plate) immersed in a sulfuric acid electrolyte and having a separator interposed therebetween. Such electrodes are typically made by applying a paste containing lead oxide(s) and lead sulfate(s) to the surface of a battery plate and electrochemically forming the paste into an active material.
Conventional pastes for use in making automotive batteries contain lead oxide (usually containing free lead in the range of 15 to 30% by weight), sulfuric acid, water and additives such as fiber and expanders. Such pastes are usually made by adding the sulfuric acid and water to a mixture of lead and lead oxide(s). As a result of the chemical reaction during mixing, a portion of the lead and PbO is initially converted to lead sulfate (PbSO.sub.4) and the resultant positive paste comprises a heterogeneous mixture of lead, lead oxide, lead sulfate and/or basic lead sulfates.
According to one known process, the paste is made by first weighing out a predetermined amount of lead oxide into a weigh hopper and dumping the lead oxide into a batch mixer, such as a mulling mixer. Dry additives such as fiber are directly added to the mixer. The resulting mixture is dry mixed for several minutes so that the fiber is dispersed throughout the oxide. Water is then added as needed to make a paste of the desired consistency. The wet mixture is mixed for a short time to wet out the lead oxide. Sulfuric acid is then added as mixing continues until the temperature peaks. The resulting paste is then cooled by evaporation of water and conduction to the mass of the mixer.
As a means of improving the ease of manufacture of batteries, a variety of conductive additives have been proposed for incorporation into the plates. Lead dioxide has been proposed as an additive for paste mixtures containing tetrabasic lead sulfate. See, Reich, U.S. Pat. No. 4,415,410 issued Nov. 15, 1983. Lead dioxide enhances positive plate formation, but provides no substantial advantage in the resulting battery because it participates in the positive plate reaction. During charging of the battery, lead sulfate is converted into lead dioxide, and the reverse reaction occurs during discharge.
The use of pre-sulfated paste materials containing basic lead sulfate, e.g., tri- and tetrabasic lead sulfates (3PbO.PbSO.sub.4.H.sub.2 O and 4PbO.PbSO.sub.4) made in dry form prior to forming the paste has also been proposed to improve the efficiency of the paste. See Malloy, U.S. Pat. No. 3,194,685, issued Jul. 13, 1965; Johnstoner U.S. Pat. No. 2,182,479, issued Dec. 5, 1939; and, Weir, U.S. Pat. No. 1,572,586, issued Feb. 9, 1926. Monobasic lead sulfate has also been used as a pre-sulfated paste material. See, for example, Voss et al., U.S. Pat. No. 3,169,890, issued Feb. 16, 1965.
Biagetti, U.S. Pat. No. 3,765,943 emphasizes the advantages of preparing a tetrabasic lead sulfate from orthorhombic lead oxide. The lead oxide starting material is mixed with aqueous sulfuric acid so that the reaction is carried out in aqueous suspension. See also, Biagetti et al., Bell System Technical Journal, September 1970, No. 49, pp. 1305-1319, wherein the pastes are prediluted with water just prior to application to the cell grids. Positive plates prepared according to such a procedure generally exhibit good performance and cycle life. However, positive plates prepared from such pre-sulfated paste mixes are difficult to form and must usually be cured for at least 24 hours before being formed. See, for example, Yarnell and Weeks, J. Electrochem. Soc., No. 126, p. 7 (1979).
Reacting lead oxide with ozone to form improved lead oxides useful as active materials in batteries is also known, as described in Parker, U.S. Pat. No. 4,388,210, issued Jun. 14, 1983, and Mahato et al., U.S. Pat. No. 4,656,706, issued Apr. 14, 1987. Parker also indicates that surface area increase is directly related to the presence of a hydrogen bonding solvent, typically water, for ozone, and that an increase in surface area is obtained with higher ozone concentration.
Metal oxides including titanium and tin oxides have also been suggested as additives for lead-acid battery plates. See, for example, Rowlette et al., U.S. Pat. No. 4,547,443, issued Oct. 15, 1985, and Hayfield, U.S. Pat. No. 4,422,917, issued Dec. 27, 1983. These additives have proven somewhat useful but fail to completely meet the need for a lead-acid battery paste capable of high performance.
Several attempts have also been made to improve the conductivity of the paste through use of persulfate treatments. For example, Reid, U.S. Pat. No. 2,159,226, issued May 23, 1939, discloses treating battery plates, before they are formed, with a persulfate by incorporating the persulfate into the paste or through use of a pickling step. When added to the paste, the persulfate is preferably incorporated into the paste and the pasted plate immersed in the solution containing the persulfate. When the persulfate treatment is performed by pickling, the plates containing lead oxide (PbO) and water may be dipped in a persulfate solution ranging from 1% up to a saturated solution. The reference specifically describes use of ammonium persulfate added in solid form to the lead oxide in preparation of the paste, or added as an aqueous solution of any strength up to a saturated solution.
Barnes et al., U.S. Pat. No. 3,398,024, issued Aug. 20, 1968, relates to the formation of a battery plate. Before the pasting operation, the grid is dipped in ammonium persulfate, sodium persulfate, or a sodium perborate solution.
Belgian Patent No. 723,018, issued Oct. 28, 1968, relates to another attempt to improve the efficiency of the charge of the positive plates by adding potassium persulfate to the mixture of lead oxide and water to prepare the active material. Particularly, this reference discloses forming a pasty mixture of lead oxide (PbO) and potassium persulfate (K.sub.2 S.sub.2 O.sub.8) by mixing the two substances with water until the lead dioxide (formed by the reaction of the lead oxide and persulfate) is uniformly dispersed throughout the resultant mixture. The addition of 10.76% by weight of the persulfate to the lead oxide is specifically disclosed. The paste of that reference does not, however, have sufficient consistency and adherence of the paste to the grid is very poor. As a result, the active material tends to shed during the formation of the battery.
Another attempt to improve the efficiency of charge of the positive plates through persulfate treatments shown in Spanish Patent No. 8801559, issued May 10, 1988, which discloses the pre-treating of each positive plate, before the formation of the battery, with a solution which contains the persulfate anion in 20 to 80 g/l concentration. The reference suggests that the persulfate ion oxidizes the active materials of the plate (PbSO.sub.4 and PbO) to conductive PbO.sub.2, reducing the total charge time by up to 50%.
Such conventional batteries and improvements thereto in accordance with prior methods fail to produce batteries capable of high power outputs. In such batteries, the energy efficiency or capacity is limited to less than 50% of the theoretical value determined according to Faraday's law. This energy efficiency is even lower at high discharge rates. Therefore, conventional SLI batteries are incapable of delivering a high power output.
High power batteries as referred to herein are batteries capable of yielding a discharged power generally In excess of one (1) watt/cm.sup.2. Such batteries are useful in applications where large amounts of power are needed in short periods of time, as is necessary in a number of aeronautical applications. While it has been suggested that high power levels can be obtained by increasing the porosity of the formed plate, high porosity weakens the plates. For example, once the porosity of conventional plates is increased to about 60%, the plate tends to lose its strength and cannot be effectively used in lead-acid batteries.
A positive paste material capable of forming a high porosity plate is thus desirable for high power output applications and heretofore has been unobtainable from the prior art teachings. The present invention addresses this problem. Particularly, the battery plates prepared in accordance with the present invention exhibit high porosity and surface area, as well as a high efficiency of formation. Such plates have good strength and are capable of high power outputs.